This invention relates to a process for electroplating steel strips or sheets for the purpose of preparing corrosion resistant steel strips which can be easily worked and show good plating appearance as well as improved overall rust prevention in the presence of a paint film applied as an undercoat, and are particularly suitable for use in the manufacture of automobiles.
Among surface treated steel strips, zinc plated or galvanized steel has found the widest variety of applications, for example, in automobiles, electric appliances, building material and the like because of its improved rust prevention. Recently, a closer attention has been paid to the drawbacks of galvanized steel and it has been desired to obviate such drawbacks. More particularly, galvanized steel strips are less compatible with paint films in which blisters often occur to substantially impair the quality of coated steel. They have inferior corrosion resistance at joints such as hemmed joints whether or not they are coated with paint.
Galvannealed steel strips, on the other hand, have found a wide variety of applications such as in automobiles, electric appliances and the like because of their improved corrosion resistance after paint coating. However, since the galvannealed steel is prepared by hot dip galvanizing steel followed by a heat treatment, the quality of the product is limited to a certain extent. In addition the plated film which is hard and brittle often exfoliates into powdery pieces during working, giving rise to so-called powdering. Therefore, much attention has recently been paid to Zn-Fe alloy electroplated steel as surface-treated steel substituting for the conventional electro-galvanized and galvannealed steel because the former steel has a combination of the advantages of the latter materials. The preparation of Zn-Fe alloy electroplated steel strips is disclosed in Japanese Patent Application Kokai Nos. SHO 56-9386, 57-51283, 57-192284, 58-52493, and 57-200589, for example. Most of these methods use a sulphate bath. However, the use of a sulphate bath at a low pH of approximately 1.0 results in a cathode deposition efficiency as low as less than 70%, which makes bath control difficult and manufacturing costly. Further insoluble anodes of Pb alloy or the like must be used for industrial production rather than soluble anodes of zinc or the like, giving rise to some problems including formation of Fe.sup.3+ ion through oxidation of Fe.sup.2+ ion in the plating bath, contamination with impurities from the anodes (particularly, lead is known to give a substantial adverse effect even at several p.p.m.), and difficult bath control. These problems are extremely difficult and costly, if not impossible, to solve. In addition, sulfate baths offer a significantly lower electrical conductance than chloride baths, for example, a fraction of that of chloride baths in the case of zinc plating, and thus require a hinger plating voltage, and hence, higher electric power and rectifier capacity at the sacrifice of economy.
The use of chloride baths eliminates the above-mentioned problems and is thus believed to be greatly advantageous for preparing Zn-Fe alloy electroplated steel strips. Such methods using chloride baths are disclosed in Japanese Patent Application Kokai Nos. SHO 57-51283 and 57-200589, for example. However, none of these methods have been commercially successful as sulfate baths have not.
Investigating the electroplating of Zn-Fe alloy from chloride baths, we found the following problems. First, the desirable iron content in the range between 10% and 30% by weight, which ensures good performance, is substantially affected by plating current density and relative flow velocity. Prior art control techniques could not provide industrial production. The aforementioned prior art methods using chloride baths only disclose particular current densities and relative flow velocities, although steady operation is not expectable at such limited points. A second problem is the propensity of forming multi-phase platings containing an .eta. phase with iron contents in the range between 10% and 30%. The presence of .eta. phase leads to inferior corrosion resistance after painting. A third problem is the quality of platings themselves which tend to be non-uniform and show poor appearance with a blackish gray color. A fourth problem is the poor adhesion of plated films resulting in exfoliation of plated films during working. Therefore, there is the need for an improved Zn-Fe alloy electroplating process capable of obviating these problems.